Many systems today, especially complex avionics systems, utilize pieces of mature technology that are combined with new technology to make the new system. It is generally more economical to use existing pieces of technology, rather than building something completely from the ground up. Reusing technology saves engineering resources that can be put to better use other parts on the system and often results in a product getting to market faster. Reusing technology saves test and verification time and resources, because a part that has already been qualified for a similar purpose generally does not need as many test cycles to verify.
Feedback control systems are commonly used in closed-loop flight control systems and other industrial or commercial products. Some well known feedback control system include lead-lag compensators and PID, or proportional plus integral plus derivative, feedback control loops. Lead-lag compensators and PID feedback control loops are well known in the art for stabilizing physical systems.
Flight control systems sometimes incorporate system components from different manufacturers that are combined to make a new system. System components that have somewhat different design specifications and parameters can exhibit undesirable behaviour when integrated together into a new system. For example, the sampling frequency for commands sent from a master computer to a closed-loop flight control system might be designed at a first frequency chosen so that the pilot or master computer has sufficient temporal granularity to accomplish a desired range of aerodynamic performance of the platform. However, the closed-loop flight control system may operate at a second much higher frequency that is designed to quickly adjust control surfaces during high speed flight with a much finer temporal granularity.
When commands are sent by the master computer at the first frequency, the closed-loop flight control system immediately adjusts the flight control surfaces in accordance to the command from the master computer at the second frequency of the closed-loop flight control system. When the closed-loop flight control system is operating at a higher frequency, or faster sampling rate, than the master computer, the closed-loop flight control system will have to wait a number of cycles between commands. This results in periodic, start-and-stop behavior of the electro-mechanical closed-loop flight control system in response to the commands from the master computer. The start-and-stop behavior acts like a “jack-hammer” on the platform and triggers structural modes at the first frequency of the commands from the master computer, causing the flight control system to exhibit undesirable behaviour.
One undesirable behavior that is triggered by the start-and-stop behavior is the creation of high current spikes that are sent to the flight control surfaces. When a command is received, the faster closed-loop flight control system immediately causes an actuation of a flight control surface, followed by the PID feedback loop attempting to stabilize the control surface to value presented in the command. This actuation by the closed-loop flight control system occurs at a high slew rate—the sampling rate of the closed-loop flight control system. This potentially results in full current commands during long slews, either with or without loads on the control surfaces.
The current spikes are followed by valleys of relatively low current usage by the flight control surfaces. This periodic demand of high current spikes followed by valleys of low current demand is inefficient from a power perspective, because it requires bigger power systems to supply power for the current spikes and also requires heavier wiring between systems. These current spikes place strains on the flight control surfaces. For example, current spikes associated with typical commands being operated on by the closed-loop flight control system can generate maximum loads on the flight control surfaces. High loads decrease the lifespan of components, requiring more scheduled maintenance and reducing the number of missions between servicing.
Another undesirable behavior resulting from the start-and-stop behavior is that lead-lag or PID feedback loop may not exhibit a desirable 40 dB/decade roll off past the specified bandwidth frequency of the closed-loop flight control system. This can lead to potential instability of the closed-loop flight control system. Attempts to attenuate the start-and-stop behavior generally result in increased phase changes that can create further instability in the system. For example, bi-quad filters on the command path can be used to smooth the command itself, but can cause unacceptable phase loss.